You are well informed about feeding introduction, Michelle, as well as observant of your baby's changing patterns. I calculate that your baby is drinking 840 ml (28 ounces) per day, which is appropriate for a child this age. He has decreased his intake only by 120 ml (4 ounces), most likely because he is in between growth spurts. There is a spurt at 3 months and another at 6 months. This decrease in formula is about 80 calories, not necessarily significant unless he is losing weight.
I suspect you will see him taking more in a very short time. Regarding your question about feeding readiness, I would recommend waiting until around 6 months to begin solid food introduction. You do not want to further decrease his calories, which naturally happens when babies start exchanging breast milk or formula for solids.
Feeding readiness happens when your son can sit erect by himself and starts reaching for food from your plate. By the way, don't be alarmed if he goes through a phase of waking again at night. This is not uncommon during periods of rapid growth, when babies get hungry more often.
Our parenting advice is given as suggestions only. We recommend you also consult your healthcare provider, and urge you to contact them immediately if your question is urgent or about a medical condition.